1. Field of the Invention
This invention relates to an apparatus and method for removing particles from the edges of semiconductor wafers in a manner that inhibits the return of such particles to the edges of the wafers. In particular, in one aspect, this invention relates to a method for removing particles from the edges of semiconductor wafers and retaining the particles away from the semiconductor wafers with an electrostatically charged material that causes a substantially unidirectional flow of particles away from the edges of the semiconductor wafers. In another aspect, this invention relates to an apparatus which electrostatically withdraws particles from the edges of semiconductor wafers in a manner that inhibits the return of such particles to the edges of the wafers.
2. State of the Art
In the electronics industry, particle contamination is an important factor in the manufacture of high density integrated circuits from semiconductor wafers. Even in relatively conventional technology using micron or larger circuit patterns, submicron size particle contamination can be a problem. Today the technology is progressing into the submicron pattern size, and particle contamination is even more of a problem. Contaminant particles larger than roughly 10% of the pattern size can create damage such as pinholes which interfere with fabrication processes (such as etching, deposition and the like) and defects of that size are a sufficiently significant proportion of the overall pattern size to result in rejected devices and reduced yield. As an example, it has been found that the minimum particle size which must be removed in order to achieve adequate yield in one Megabit chip (which has a pattern size of one micron) is about 0.1 microns.
Filtration (of air and liquid), particle detection, and contaminant removal are techniques which are used in current contamination control technology in order to address the problems outlined above. For example, semiconductor fabrication is often conducted in clean rooms in which the air is highly filtered, the rooms are positively pressurized, and the personnel allowed into the room are decontaminated and specially garbed before entry is allowed.
Another previously developed method of particle removal is to place the surface to be cleaned in a chemical bath and then use an ultrasonic or a megasonic sound agitating system. In this system, the ultrasonic or megasonic sound is induced into the liquid which shakes the chemical bath and attempts to remove any particles. This has been somewhat effective against relatively large particles, but ineffective against small particles.
Another cleaning method which has been previously used is to blast the surface to be cleaned with a fluid in order to blow or wash the particles from the surface. One such method utilizing water is disclosed in U.S. Pat. No. 4,027,686, to Shortes, et al. and assigned to Texas Instruments, Inc. Other fluids, including air, have been used with varying degrees of success. These methods have proven to be effective against large particles, but relatively ineffective against particles in the 0.1 micron or smaller range.
Another previously developed method involving application of force to the particle exploits the mass of the particle by the use of gravity or centrifugal force. This method requires the particle to be of sufficient size to be pulled or thrown from the surface. When the particles are very small or have little mass, the forces may not be high enough to remove them. Therefore, this method is generally ineffective against submicron sized particles.
Still another line of previous attempts to remove particles is to convert them into nonparticles. This requires a knowledge of the chemistry of the particles, or a universal solvent (one that would dissolve all particles). This method uses a chemical specifically designed to attack and dissolve the particles but not the substrate they are on. It is necessary to know what the particles to be removed are and, therefore, the appropriate chemical. Since it is unlikely that the composition of all the particles will be known, it is difficult to remove all the particles with a chemical.
Another prior method to remove particles involves particle and surface interaction. This is an attempt to make the particle and the surface repel each other through the use of electricity. The problem encountered with this method is that a high electrical charge is required and, thus, it has been found to be undesirable.
One of the most common methods of particle removal involves using a brush scrubber. In this method, a brush is used to wipe the particles from the surface. It is necessary to compromise the stiffness of the brush bristles so that they are stiff enough to remove the particles yet soft enough to not damage the surface. This method is effective against relatively large particles only and is relatively ineffective on the edges of the wafers.
A final method of removing particles has been an attempt to adhere the particles to a removable film. A polymer gel or liquid is applied to the surface in an attempt to soak up all the particles. After the polymer has dried, and hopefully drawn the particles into it, the polymer is peeled off to remove the particles. This method, as with the other methods listed above, works only with relatively large particles and doesn't address the problem of particles situated at the edges of the wafers.
As the particle size decreases the particle weight become less significant as compared to other adhesive forces binding the particle to the surface which it contaminates. Removal of such small particles can potentially damage the substrate. In general, it has been found that submicron particles are the most difficult to remove. Many of the processes developed to clean integrated circuits, such as ultrasonic agitation, are not effective for micron and submicron particles and indeed, sometimes add contaminants to the substrate.
As alluded to above, another handicap of these previously developed physical removal processes is the fact that they can damage the surface being cleaned as well as introduce other particles while trying to remove them.
Additionally, prior art methods of removing particles from semiconductor wafers have focused almost exclusively on the removal of particles from the front and back surfaces of the wafers. However, by leaving particles on the sides or edges of the semiconductor wafers, it is possible for these particles to migrate to the front or back surface of the wafer by, e.g., physical, mechanical, electrostatic or other such means, and there interfere with the processing of the wafer.
Accordingly, there is a great need for a removal technique which allows for the removal of particles from the edges of semiconductor wafers. Such a removal technique should also provide a driving force sufficient to effectively remove particles from the edges of semiconductor wafers without any substantial harm to the substrate material. Moreover, an acceptable removal technique should provide a minimum level of cleanliness in a reliable fashion.